Fig. 597.--Murray loop method of fault location with Leeds and Northrup fault finder: Case II, where the good and bad wires are unequal. The figure shows the connections. It is the ordinary Murray loop and it is evident that the resistance a, to the fault will be obtained from the formula a = (A ÷ 1,000) × r, where r is the resistance of the loop, and A is the reading of the contact C on its scale. The distance d, to the fault is obtained from the formula d = Ar ÷ (1,000 × M), where M is the resistance per mile of the faulty wire.
EXAMPLE--A wire having a resistance M of 16.46 ohms per mile is grounded. This wire was looped with a wire of unknown resistance and the total resistance of the loop r was measured and found to be 54.07 ohms. Connections were made as in the figure, and the reading A was found to be 332. Substituting these values in the above formula: d = (332 × 54.07) ÷ (1,000 × 16.46) = 1.09 miles.
Ques. How may the distance from 2 to the fault be determined in knots or miles.
Ans. Divide Y by resistance per knot or mile.
The Varley Loop.--This is a method of locating a cross or ground in a telephone or telegraph line or other cable by using a Wheatstone bridge in a loop formed of a good wire and the faulty wire joined at their distance ends. One terminal of the battery is grounded and the other connected to a point on the bridge at the junction of the ratio arms. The rheostat arm then includes the resistance of the rheostat plus the resistance of the fault, while the unknown arm includes the resistance of the good wire plus the resistance of the bad wire beyond the fault. When the bridge is balanced, the unknown resistances may be readily determined by a simple equation.
Fig. 598.--The Varley loop test. The diagram shows the various connections. X and Y are the resistances of the cable between the fault and the points 1 and 2 respectively. L is the resistance of the good and bad cable or X + Y.
In making the Varley loop test, the resistance of looped cable or conductors is measured, and then connected as in [fig. 598]. Close the battery key and adjust R for balance.
When earth current is present, the best results are obtained when the fault is cleared by the negative pole, and just before it begins to polarize. If X be the resistance from 2 to the fault, then
X = (L - R) / 2
also, X divided by the resistance of the cable or conductor per knot or mile gives the distance of fault in knot or miles.
When the resistance of the good wire used to form a loop with the defective wire, together with that portion of the defective wire from the joint to the fault is less than the resistance of the defective wire from the testing station to the fault, the resistance R must be inserted between point 1 and the good conductor, the defective wire being connected directly to point. The formula in this case is
X = (L + R) / 2
Figs. 599 and 600.--Varley loop method of fault location with Leeds and Northrup fault finder. This method may be used as a check on the Murray methods. Connect the faulty wire to 1, and measure the resistance of the loop. Then throw switches as shown in the [fig. 600]. Let: a = resistance to fault, d = distance to the fault in miles, M = resistance of the faulty wire per mile, r = resistance of the loop, R = resistance of the coil R, or 100 ohms, T = A ÷ (1,000 - A) to be read from the table. From the Wheatstone bridge relation: a = (r - 100T) ÷ (T + 1), and d = (r - 100T) ÷ (T + 1)M.